Recently, this type of data communication apparatus performs data communication using a V.34 modem (28.8 kbps) which is specified by the ITU-T. The ITU-T also recommends T30 ANEXF (so-called Super G3) as facsimile communication standards using the V.34 modem for facsimile machines. A pre-communication protocol for facsimile communication is carried out according to the standards, after which communication of image data is executed.
Such a communication protocol will be explained based on the sequence chart illustrated in FIG. 13. FIG. 13 is a control signal chart for a pre-protocol for facsimile communication according to prior art.
Referring to FIG. 13, reference character 13a denotes a communication protocol for selecting a modulation mode from among a V34 half duplex, V34 full duplex, V17 half duplex, etc. Reference character 13b denotes a communication protocol for implementing line probing to check a line and determine various kinds of parameters. Reference character 13c denotes a communication protocol for modem training. Reference character 13d denotes a communication protocol for setting a modem parameter. Reference character 13e denotes a communication protocol for exchanging a facsimile control signal. Reference character 13f denotes a data communication protocol for the primary channel. The upper side in the diagram is a sequence for a caller modem, and the lower side is a sequence on an answer modem, and the sequences progress from left to right.
The above communication protocols will be discussed specifically.
First, in the communication protocol 13a for selecting a modulation mode and communication protocol, which permit communication between a caller modem and an answer modem, are selected through a V.21 modem (300 bps, full duplex) after a line connection is established. A facsimile machine using a V.34 modem selects a V.34 modem as the modulation mode and facsimile communication as a communication protocol.
Then the communication protocol 13b for line probing checks the line by transmitting a line probing tone from the caller modem and receiving it on the answer modem, and selects a training parameter based on the result of the line inspection.
In the communication protocol 13c for modem training, the caller modem sends training signals based on the training parameter selected under the line probing communication protocol 13b, while the answer modem receives the training signals, learns a filter coefficient for an adaptive equalizer for compensating the line characteristic and checks the reception quality of the training signals.
In the communication protocol 13d for selecting a modem parameter, modem parameters are negotiated between the caller modem and answer modem in full duplex communication as 1200 bps, and an optimal modem parameter is selected from the modem parameters preset in the apparatus, the result of the line inspection and the inspection of the reception quality of the training signals.
The communication protocol 13e for a facsimile control signal is an ordinary facsimile protocol to execute negotiation of facsimile control signals NSF, CSI, DIS, TSI, DCS, CFR, etc. in full duplex communication at 1200 bps.
In the data communication protocol 13f, the caller modem sends image data and the answer modem receives the image data, in half duplex communication at 2400 bps to 28.8 kbps. In the case of performing communication at the maximum communication rate of 28.8 kbps, image data can be communicated in approximately three seconds per a sheet of paper of size A4.
The aforementioned modem performs communication in accordance with the training parameter selected under the communication protocol 13b for selection under the communication protocol 13d for selection of a modem parameter. To compensate the line characteristic, the receiver modem executes communication using the filter coefficient that has learned in the modem training 13b. This ensure optimal data communication according to the line quality.
The above-described prior art structure involves five channels of a pre-protocol before starting sending image data after line establishment, and thus requires about 7 seconds. By contrast, since electric transmission of a single sheet of image data at the maximum communication rate of 28.8 kbps takes about 3 seconds, the pre-protocol requires over 60% of the entire time of 11 seconds required for transmission of one sheet of an original including the post-protocol of about 1 second. This time needed for the pre-protocol gets greater as the number of transmission/reception lines increases, and generates wasteful time and communication cost.